1. Field of the Invention
This invention relates to a process for descaling cold-rolled and annealed stainless steel strip by electrolytic pickling, and more particularly to a continuous process for removal of scales on the surface thereof in a short time.
2. Description of the Prior Art
So far known methods for continuously descaling cold-rolled stainless steel strip include, for example, as a preparatory step, salt treatment with a molten alkali salt consisting essentially of NaOH or Na.sub.2 CO.sub.3, or electrolytic treatment in a solution of neutral salt, such as Na.sub.2 SO.sub.4 or NaNO.sub.3, followed by, as main step, the immersion in an aqueous solution of sulfuric acid, nitric acid added hydrogen fluoride, or nitric acid, or electrolytic treatment with an aqueous solution of sulfuric acid or nitric acid. From these methods consisting of immersion or electrolysis which have been disclosed in Japanese Laid-open Patent No. 59-59900a, suitable one is generally applied depending on the extent to which descaling can be made, varying with a kind of steel or annealing conditions of steel to be descaled.
Also in these complex processes, however, it takes a long time for full descaling to be accomplished, and this is still a cause for limited efficiency in production of cold-rolled stainless steel strips. It is troublesome to regulate concentrations of many different salts and acids. Salt treatment is inevitably accompanied by substantial supplement of salt carried away with descaled steel strips.
In an attempt to solve the above-mentioned problems, the inventors made previously a proposal (Japanese Laid-open Patent No. 049197/1987), which has enabled them to descale steels when comparatively difficult to do so, such as SUH409 obtained by annealing at 900.degree. C. or higher temperature on a CAL continuous annealing line, under the condition of 5% of H.sub.2 with residue of N.sub.2 and dew point of 20.degree. C. below zero. It however is of the socalled two-step-electrolysis system requiring two electrolytic cells and two electrolytic solutions and particularly consisting of electrolysis with a high concentration sulfuric acid solution of 900-1250 g/l in sulfuric acid concentration, followed by with a nitric acid solution containing HCl, FeCl.sub.3, NaCl, or the like. Accordingly, it was disadvantageous in having a room to be improved or simplified compared with the one electrolytic-solution descaling technique.